Water that runs off from these green acres typically picks up a load of fertilizers, pesticides and other potentially toxic chemicals, and washes them—via sewers or directly—into lakes, rivers, streams and even the ocean. Once there, joined by similar runoff from agriculture, the chemicals can drive a host of environmental problems, ranging from dead zones to contaminated fish.

Previous estimates of how much water pollution derived from the suburbs was based simply on rainfall. But horticulturalist Lorence Oki of the University of California, Davis, and his colleagues found that sprinklers and other irrigation techniques also led to significant runoff that, in some cases, carried more pollution with it from the eight neighborhoods studied in Sacramento and Orange counties than runoff after a rain storm.

Pesticides—both organophosphates and pyrethroids—were found in all the water samples, which were collected on a weekly, biweekly and monthly basis for more than two years. The majority of these pesticides, 60 percent, were purchased to control ants, according to a survey by the California Department of Pesticide Regulation.

But there is a simple solution to lawn runoff. A study at the University of Michigan, published online August 14 in the journal Lakes and Rivers Management , found that Ann Arbor's ban on phosphorous fertilizers for grass led to a 28 percent drop in the pollutant's levels in nearby Huron River.

Keeping lawn-care chemicals out of U.S. waters may be as simple as banning them—or at least cutting down on running the sprinklers right after applying fertilizers or pesticides.